As people have discovered, Kobo have release firmware version 4.2.8110. This is only being released for the Kobo Aura ONE and Kobo Aura Edition 2. It should be safe on the other devices, but it's up to you to take the risk. Hopefully something will be released for the other devices early in the new year.

The following is a list of changes Kobo posted in the beta list plus things I can remember seeing:

Wishlist added to the store.

Battery drain improvements for Kobo Aura ONE. Some of these changes affect the other devices. I haven't had any unusual battery drain on any of my devices with the betas for 4.2.

Auto Brightness improvements for Kobo Aura ONE.

Made many improvements to help ensure your library state and contents are correct.

Added a Repair option available under [Settings > Device information], in case you still seem to be missing a book.

Lots of fixes and changes in the Overdrive support.

There has a release of the Kobo Desktop. the Windows version is 4.2.6122 and the macOS is 4.2.4917. The changes include:

Kobo Desktop visual refresh – new app icon and colour palette

Made many improvements to help ensure your library state and contents are correct

Added a Repair option available under [Account > Repair Kobo Account], in case you still seem to be missing a book

The Repair option mentioned is to fix problems with purchased books. When this is used, all the purchased books are removed from the device and downloaded again. It will take a while if you have a lot of books. You will be prompted before it starts.

The calibre driver has been updated to accept this firmware version. This will be in the next release of calibre. As there are no changes needed other than bumping the version number, you can safely use the "Attempt to support newer firmware versions" option in the driver until the release is made.